The present invention relates to the lighting arts. It is particularly applicable to the fabrication of high-brightness gallium nitride (GaN) based light emitting diodes (LEDs) and LED arrays, and will be described with particular reference thereto. However, the invention also finds application in connection with other types of LEDs and in other LED applications.
LEDs, particularly those fabricated from gallium nitride (GaN), aluminum nitride (AlN), indium nitride (InN), and various alloys and layer combinations thereof, are suitable for illumination applications in the ultraviolet and blue wavelength ranges. Additionally, GaN-based LEDs which are coated with a wavelength-converting phosphor are suitable for producing white or selectably colored light for illumination. Such LEDs have a number of advantages over other types of illuminators, including compactness, low operating voltages, and high reliability.
However, GaN-based power LEDs for lighting applications suffer from low luminous output. A state-of-the-art GaN-based LED presently dissipates about 5 watts while generating about 100 lumens. In contrast, a typical incandescent light source dissipates about 60 watts while generating about 1,000 lumens of light output. Heat dissipation is a limiting factor in the luminous output and reliability of power LEDs. Although heat sinks provide for heat removal in LED-based lighting systems, there remains a need for improved heat removal from the active region of the LED die to the heat sink.
In a conventional GaN-based LED package, the active GaN layers are arranged on a sapphire or other transparent substrate in a flip-chip orientation in which the active GaN layers are bonded to a silicon or other thermally conductive sub-mount which in turn is supported on a lead frame or a printed circuit board. The silicon sub-mount is insulating and includes bonding pads that connect with electrodes of the LED die during the flip-chip bonding. The bonding pads of the silicon sub-mount are electrically connected with the lead frame or traces of the printed circuit board by wire-bonding. Light emitted by the GaN layers is transmitted through the transparent LED substrate, while heat produced by the GaN layers is conducted through the silicon sub-mount to a heat sink integrated into or associated with the lead frame or printed circuit board.
The conventional GaN-based LED package suffers from a number of disadvantages. Packaging is complex due to the separate steps of bonding the GaN layers to the sub-mount, bonding the sub-mount to the associated support, and wire-bonding the bonding pads to the lead frame. The package is incompatible with wafer-level processing typically employed in other areas of the electronics industry, which impacts production throughput and yield. Although some wafer level processing can be performed on the epitaxial GaN wafer prior to dicing, certain processes such as die encapsulation cannot be performed prior to dicing. Thermal conductivity, while improved by using the silicon sub-mount, is still non-ideal. The overall package is bulky.
The present invention contemplates an improved apparatus and method for forming the same that overcomes the above-mentioned limitations and others.